by Dr. L. E. Leguire
The cones are light sensitive cells in the back of the eye (retina) responsible for vision during the day and in bright lights. The cones are responsible for fine detailed central vision – for vision when you look directly at something like a person’s face. Cones are needed for color vision and for fine detailed vision – like looking at really small objects (e.g., ants) and for seeing in bright lighted environments – like a sunny day.
The rods are light sensitive cells (photoreceptors) in the back of the eye (retina) responsible for seeing at night and under very low light levels. Rods are responsible for seeing at night when there is only a little light, like moon light. Rods are responsible for side vision. Rods can only see black-and-white; rods cannot see colors.
When a person has a cone – rod degeneration, first the cone cells are affected and then the rod cells are affected. When the cones are first affected the patient has a hard time seeing straight ahead and may have to look off to the side to see things more clearly. Sometimes these patients have a dark spot right in the middle of their vision and only have side vision. This dark spot is called a “scotoma”, an example of which can be seen under the Simulations page (similar to a Macular degeneration). Patients with cone–rod degenerations have defective color vision and may confuse certain colors like blue and green. Some patients with cone-rod degeneration become very sensitive to lights and want to wear sunglasses even inside the house.
As the cone-rod degeneration becomes worse, the rod cells may start to become affected. This leads to a bigger dark spot in the center of vision and color vision gets worse. As the disease gets worse, side vision may start to become affected and the patient may become night-blind. During the late stages of the cone-rod degeneration the patient may be able to see only large and high contrast objects (large black and white objects) and may not be able to read or drive a car.
Cone-rod degenerations may affected immediate family members who have the same mother and father (in other words sisters and brothers of the affected family member). There may be as high as a 50% – 50% chance of sisters and brothers affected. If there is a strong family history (Dominant), 50 % of all family members including sisters and brothers and sons and daughters may be affected. If there is no family history of very poor vision there is very low chance that sons or daughters of the affected person will be affected (almost same as in the general population). However, if there is no family history of very poor vision there is still a possible 25% chance that sisters and brothers of the affected person could have the cone-rod degeneration.
A cone-rod degeneration is very similar to a macular degeneration, since the macula (back part of the eye) contains both cone and rods cells that become abnormal affecting both day light vision and night vision. Rarely does all vision disappear in patients with cone-rod degenerations.
Also see info about cone rod degeneration and the genes involved under Stargardt’s disease.